如何将ListBoxFor值保存到MVC5中的数据库

Question

I want to save all listBoxFor values to the Database... when posting the form i have this error saying Object not Set to an Instant of an Object, Can anyone help me to figure out what the problem is?

This is all the code

This is the listbox code that contain all the values:

@Html.ListBoxFor(model => model.SelectedRecipientId, new MultiSelectList(Model.Recipients, "ID", "RecipientNumbers"), new { id = "recipientList", style = "width: 250px; height: 160px;", name = "recipientList" })

This is my View model

public class SendViewModel
{
    public SendViewModel()
    {
        ScheduledDate = DateTime.Now;
    }

    public int Id { get; set; }
    public string Message { get; set; }
    public DateTime ScheduledDate { get; set; }

    public IEnumerable<SelectListItem> Times { get; set; }
    public string[] SelectedRecipientId { get; set; }

    [Required]
    public IEnumerable<SelectListItem> Recipients { get; set; }
}

This is the model that I am trying to write to

public class SentMessage
{
    public int Id { get; set; }
    public string Message { get; set; }
    public string SenderId { get; set; }
    public DateTime SendDate { get; set; }
    public int RecipientCount { get; set; }
    ...

    public virtual ICollection<MessageRecipient> Recipients { get; set; }
}

This is my Post Controller

if (ModelState.IsValid)
{
    if (model.ScheduledDate == DateTime.Now)
    {
        _messageService.SendMessage(User, SenderId, model.Message, model.SelectedRecipientId);

        return Redirect("/UserAccount/SendMessageConfirmation");
    }
}

And lastly this is the message service parameter collection

int SendMessage(int userId, string senderId, string message, IEnumerable<string> recipients);

This is the Signature that save all the data from the view to the database and the IEnumerable<string> recipients is the parameter responsibl for getting all the values from the listbox.

How do I go about it so that it will be able to save to the database.

This is the Error stack Trace

Server Error in '/' Application.

Object reference not set to an instance of an object.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NullReferenceException: Object reference not set to an instance of an object.

Source Error:

Line 68: Line 69: Recipients Line 70: @Html.ListBoxFor(model => model.SelectedRecipientId, new MultiSelectList(Model.Recipients, "ID", "RecipientNumbers"), new { id = "recipientList", style = "width: 250px; height: 160px;", name = "SelectedRecipientId" }) Line 71: Line 72:

Source File: c:\\Development\\Projects\\SMSXChange\\SmsXchange\\XChange.MVC\\Views\\Message\\Send.cshtml Line: 70

Stack Trace:

[NullReferenceException: Object reference not set to an instance of an object.] ASP._Page_Views_Message_Send_cshtml

Answer 1

Thanks for the error details,

Although the thing i mentioned in my comment is correct that both " HTML Element Name " and " Model Property Name " should be similar to post the value but here the problem is something different for the error you are getting.

Make sure that you are filling your Model.Recipients property correctly before rendering the view. Your action from where you are calling the view should assign correct value for Model.Recipients property. Your property is null hence it is throwing an object reference error.

In fact the error should come on initial load of the form and not after posting it. If still it is coming after posting the form then there is following possibility. As you have added ModelState.IsValid check it seems this is not getting satisfied and there is some validation error which is causing the Post action method to render same view again (Provider you have are not doing anything else in else part) and when the view gets loaded again it doesn't have the select list filled as you haven't added that code in post method. Hence the possible solution would be to fill the Model.Recipients again on POST action if there is any validation error. I hope you got my point